1. Field of the Invention
The present invention relates to a drop-out detecting circuit for detecting drop-outs of a reproduced signal, which is suitable for use in a high density magnetic recording apparatus for backing up data.
2. Description of the Related Art
In a magnetic recording apparatus such as a high density magnetic recording apparatus for data backup, for instance, it is required to secure reproduction reliability when data are reproduced from a magnetic tape through a magnetic reproducing head. For this purpose, when data are recorded on a magnetic tape by a magnetic recording head, the data recorded on the magnetic tape are reproduced and then checked, immediately after the data have been recorded, by the magnetic reproducing head installed just behind the magnetic recording head. More specifically, when the level of the signal reproduced by the magnetic reproducing head is lower than a predetermined level (when a drop-out is detected), the reproduced signal is isolated as a read error. Data which are the same as the isolated reproduced signal are recorded again (rewritten) on another location on the magnetic tape. Therefore, it is possible to prevent the drop-out portion of the data recorded on a magnetic tape from being used.
In the recording apparatus as described above, a drop-out detecting circuit detects the fact that the level of the signal reproduced by the magnetic reproducing head is lower than that of a reference voltage level, that is, there exists a dropout of the reproduced signal.
FIG. 8 shows a prior art drop-out detecting circuit, by way of example, and FIG. 9 shows a timing chart of the same circuit.
In the high density magnetic recording apparatus for data backup, data are divided into blocks and recorded on a magnetic tape through the magnetic recording head. The signal reproduced from the magnetic tape is first amplified by a preamplifier (not shown) and then inputted to a non-inversion input terminal of a comparator 1 as read data A. The read data or reproduced signal A is compared with a reference voltage signal B inputted from a reference voltage supply 2 to an inversion input terminal of the comparator 1 to detect the presence of any drop-out. In response to the leading edge of an output signal C (drop-out signal) of the comparator 1, a monostable multivibrator 3 is triggered to generate a pulse signal D with a predetermined pulse width. The width of this pulse signal D with respect to time is determined to be longer than the longest data time period. The pulse signal 25 D having the above-mentioned pulse width is outputted to an AND gate 4. On the other hand, an output signal of the preamplifier is passed through a low-pass filter (not shown) and then differentiated through a differentiation circuit (not shown). The differentiated signal is compared with a zero cross point through a zero-cross comparator (not shown), and then provided as read data E. This read data E is applied to the AND gate 4. Therefore, the read data E from which the drop-out portion is removed on the basis of the output signal D of the monostable multivibrator 3 is outputted through the AND gate 4 to a demodulator (not shown).
In the above-mentioned prior art drop-out detecting circuit, however, the level of the reproduced signal A supplied to the comparator 1 fluctuates due to fluctuations of the output level of the magnetic reproducing head and the gain of the preamplifier. Therefore, in order to set the level at which the drop-out is detected (the reference voltage level B of the reference supply voltage 2) to a predetermined ratio of the maximum level of the reproduced signal A, it is necessary to regulate the gain of the preamplifier and the level of the reference voltage B of the reference voltage supply 2. This results in an increase in manufacturing cost due to increases in the number of the electrical parts and the steps of circuit adjustment or assembly.